For those who don't know, the author of this book is Johnn Four, proprietor of the weekly "roleplaying tips" newsletter. Many of the points presented therein will seem familiar to you if you are a roleplayingtips subscriber (been one for a long time...), but it is still nice to have all those ideas streamlined and codified, and put in a convenient format. He also has a column in Dragon with some of the same sorts of tips.
As for being d20 -- the Johnn Four would have sold the product to me faster than d20 would have (in fact, I didn't pick it up until I saw who wrote it!), but I imagine that point is less than universal. There's lots of d20 "crap" out there, and this goes doubly for the PDF d20 market (here's hoping you guys can get this out as a print version!)
But the d20 references are light. It is nice for d20 GMs that he speaks in terms of levels and suchnot when describing considerations when designing NPCs, but with the exception of the adventure, the book is really universal in its utility. Any GM who needs some brushing up on their NPC skills could use this book, regardless of system!